Electron discharge device



Mal-ch24, 1936. H. c. THOMPSON 2,035,003

ELECTRON DISCHARGE DEVICE Filed Aug. 31, 1953 i i i i i i 2 "1 1.

I HI [I vlea n I-Efl/VPL/F/ER.

K s k T 73 s E J lg s N N g INVENTOR fill/PR) c. 711M290 ATTORNEY Patented Mar. 24, 1936 UNITED STATES ELECTRON DISCHARGE DEVICE Harry 0. Thompson, New York, N. Y., assignor to Radio Corporation oi America, a corporation of Delaware Application August 31, 1933, Serial No. 687,565

9 Claims.

This invention relates to electron discharge tubes and more particularly to amplifier tubes in which under some conditions electrons may pass the plate and strike the walls of the glass bulb.

Amplifier tubes of the screen grid type may be designed to have characteristics which adapt them for use in high gain circuits. Such tubes, designed for high impedance and constructed in the usual way, have been found to have a tube 10 impedance which when measured under ordinary test conditions, was what would be expected from the geometry and dimensions of the tube. However, such tubes, when used in high gain amplifier circuits with high impedance in the plate circuit sometimes gave an output which corresponded to the output of a tube of much lower impedance, indicating that for some reason the effective impedance of the tube had been substantially diminished. This diminished impedance resulted in circuit losses which amounted in high gain circuits to such a reduction in output as to result in unsatisfactory operation of the tube in such circuits.

An object of the invention is to provide a method and means for suppressing the circuit loss due to conditions which make the effective impedance of the tube less than the impedance measured under ordinary test conditions.

Another object of the invention is to provide an electron discharge tube having on the inner walls of the glass bulb a surface of such a nature that secondary electron emission from the walls is negligible under normal operating conditions.

In accordance with my invention I provide on the inner wall of the glass bulb a surface, preferably a matte or difiusing surface, of such a nature that under operating conditions the number of secondary electrons emitted is less than the number of primary electrons striking the wall, and the ratio of secondary electrons emitted by that surfare to the incident electrons is main tained at a value less than one to one. It is my belief that the decrease in output observed when the tubes constructed in the usual way were 4 used in high gain circuits is in some way dependent on secondary electron emission from the inner wall of the glass bulb, but in any case tests show that a tube which has on the inner wallof the bulb a matte surface with low secondary electron 50 emissivity gives improved operation in a high gain circuit.

In general, the desired kind of surface, which is a poor emitter of secondary electrons, may be produced by covering the inner wall of, the glass 55 bulb of the electron discharge tube with a material which has a lower secondary electron emission efliciency than glass, or with a porous or rough material which by virtue of its physical structure reduces the secondary electron emitting efiiciency of the surface, or with such a poor- 5 1y emitting material in a porous form, thus combining the two desired effects.

For a better understanding of the present invention reference may be had to the following description taken in connection with the accom- 10 panying drawing, in which,

Figure 1 is a front view, partly in section, of a glass bulb treated in accordance with this invention.

Figure 2 is a front view, with parts broken away, 15 of an electron discharge tube with a glass bulb such as is shown in Figure 1.

Figure 3 is a diagram of a representative type of circuit in which tubes made in accordance with this invention may advantageously be used. 20

A tube embodying my invention may be made with a glass bulb such as shown partly in section in Figure 1. This glass bulb has an intermediate part or zone of its inner wall covered with a coating or thin layer 2 of some material, such as 5 finely divided carbon, which produces a coating with a slightly rough or porous surface, and makes the electron emitting efiiciency of the coated surface less than that of the usual inner wall of the bulb. The coated zone or band on the 30 bulb is in registry with and surrounds the electrode assembly when the bulb is made into a radio tube.

Figure 2 shows a radio tube having character: istics which make it suitable for use in high gain 35 circuits, and of the type disclosed more in detail in the application of Terry M. Shrader, Serial No. 605,879, filed April 18, 1932, and assigned to the same assignee as this application. The electrode assembly of this tube is enclosed in a highly evacuated glass bulb l which has the usual base 3 at the bottom, contact cap 4 at the top, and the usual re-entrant stem 5 sealed into the neck of the bulb. The stem 5 carries the electrode assembly, which comprises a unipotential cathode 6, a first or control grid 7 connected to the cap 4,

a second or screen grid 8, a third or regulator grid 9, and an anode or plate l0, preferably an imperforate cylinder of carbonized sheet metal, such as nickel, surrounding and coaxial with the grids and cathode and also supported on the stem 5. The electrodes are spaced as usual by mica spacers II and I2. As shown on the drawing, the anode is shorter than the cathode, and hence electrons from the cathode, particularly to from its end portions, may pass the ends of the anode and reach the bulb walls. A deep metal cup l3, carried on rods l4, acts as a top shield, and a steadying disc 15 of mica on the upper ends of the rods I l fits into the dome top of the bulb I and holds the electrode assembly steady in the bulb.

If a tube of the structure shown in Figure 2, with the bulb having an inner surface such as is found in tubes made in the usual way, is used in a high gain amplifying circuit, the output of the tube may under some conditions be erratic and lower than would be expected. This dimculty arises because the effective or dynamic impedance of the tube seems to be lower than the static impedance, as measured under usual test conditions. As the dimensions and relative length of the electrodes of the tube are in general fixed by the characteristics which the tube must have for commercial reasons, this diificulty must be overcome without making structural changes in the electrode assembly.

In accordance with my invention the bulb of the tube has, as shown in Figure 2, a coated or covered zone 2 surrounding and in registry with the electrode assembly and preferably of a width greater than the length of the plate In so that the ends of the anode or plate are overlapped to some extent by the coating. One satisfactory coating for the zone 2 is finely divided carbon. The carbon coating may be applied by introducing into the bulb the smoky flame of a turpentine torch and thereby smoking the interior of the bulb until carbon black is deposited as a rough porous coating of fiufiy carbon over practically the entire inner surface of the bulb so that the latter becomes nearly opaque. The excess carbon may then be removed from all of the bulb interior, except the intermediate zone 2, by means of suitably shaped wipers or brushes. The carbon coating is left as an annular band positioned on the inner wall of the bulb to surround the electrode assembly, and insulated from all of the electrodes by'a zone of clean glass on each side of the carbon band. 2. The anode to ground capacity of the tube is much less with the insulated carbon band 2 than with the band grounded. Another carbon coating method which has been found to be particularly suitable for commercial use, is to spray the inside of the glass bulb I with a suspension of carbon such as commercial lamp black in alcohol, dry, and then remove the carbon coating from the neck and top of the bulb by power driven rotating brushes inserted in the bulb.

The dry carbon coating remaining on the bulb wall should be slightly translucent, and may to advantage be of such thickness that a yellow object, such as a yellow lead pencil, held inside the bulb and viewed through the carbon coating in ordinary daylight, can be seen faintly. The coating should preferably be uniform and not spotty or streaked. The surface of the carbon coating thus produced is highly roughened, with a very great number of fine pores or indentations, and is similar to the matte surfaces commonly used in optics to give complete diffusion, that is, a surface void of any specular reflection. A suspension having the following ingredients and proportions has been found to be commercially satisfactory.

Carbon (lampblack) grams Methanol 2700 c. c.

To prepare the suspension, the above quantities of materials are ball-milled with fifty inch smooth fiint stones for four hours in a one gallon ball mill rotated at 45 R. P. M. and then drawn 05 for use.

It has been found that a carbon coating prepared and applied as described above is of such high resistance that it is, in marked contrast to a coating of graphite, for example, and has less electron emissivity than glass, hence coating the glass walls of a bulb with such a suspension causes a marked reduction in the secondary electron emission which comes from the bulb walls when primary electrons strike the bulb. The benefits of the carbon coating are not limited to-any specific size or shape of bulb, but may be obtained with various types of radio tubes.

The desired results may also be secured by treating the bulb surface throughout the coated zone 2 with materials other than carbon. For example, a coating of finely divided aluminum oxide, applied as an aqueous suspension will give much the same results as finely divided carbon. As aluminum oxide is an insulator, this coating is also a coating insulated from the electrodes.

In Figure 3 is shown a circuit diagram of one circuit application of a radio tube embodying the present invention. The tube which receives radio signals of intermediate frequency from the I--F input circuit I6 is diagrammatically shown, and is of the type illustrated in Figure 2, with the third grid 9 connected to the cathode to act as a suppressor grid. The input circuit I6 and the output circuit I! connected to this tube may be designed in the usual way for high frequency amplification. The present invention, however, relates particularly to the amplifier part of a circuit such as is shown in this figure. When the tube used in such a circuit has an inner bulb output circuit l1 and consisting of a virtual resistance in series with a virtual capacitance. For convenience of discussion, this virtual shunt circuit is indicated in dotted lines in Figure 3 r as a virtual resistance I 8 and a virtual capacitance l9. Such a shunt obviously reduces the dynamic plate impedance of the tube and likewise the volume output of a receiver employing such a tube. Measurements have shown that a tube of the type shown in Figure 2 with its bulb walls having the usual surface had at radio frequency a dynamic plate impedance of 200,000 ohms or less as calculated from actual gain through a known load impedance, while a tube similar in every respect except that it had the carbon coating 2 on the inner wall of the bulb had in this circuit a dynamic plate impedance of about 800,000 ohms, approximately the same as its static impedance as measured by the usual tests.

While I do not wish to advance any particular theory of operation, I believe that in a tube of the type shown in Figure 2 some of the electrons from thecathode 6 may pass the ends -of the plate l0. These stray electrons may then go to the back of the plate, or may go to the bulb if the bulb wall is sufliciently positive. The stray electrons which go to the bulb may cause secondary electron emission from the bulb wall, and it seems probable that the drop in effective impedance of the tube is in some way dependent on the secondary emission from the bulb walls. Tests have been made with a tube of the type shown in Figure 2 which was constructed in the usual way, but in addition had on the outer surface of the plate l0 and also on the inner wall of the glass bulb opposite the electrode assembly a thin coating of willemite, a material which fluoresces when bombarded by electrons. When the tube functioned normally and gave the expected output at 250 volts on the plate, two bright bands appeared on the back of the anode, indicating that a stray electron stream from the cathode passed around the edges of the plate and impinged on its rear or outside surface. When a positive potential of 250 volts was applied to the outside of the glass bulb through a high resistance, the two bright bands shifted from the back of the anode to the wall of the bulb and coincident with this shift a definite drop occurred in the output of the tube. When the positive potential of 250 volts was removed the bands remained on the bulb and the circuit loss continued. This test indicated that stray electrons which pass the plate may go to the bulb when the wall is positive, and that under such conditions the output of the tube may be lower than when the stray electrons do not reach the bulb.

It seems probable that when a bulb wall with the usual glass surface is bombarded by stray electrons the average number of secondary electrons leaving the wall is greater than the average number of primary electrons striking it so that the bulb wall goes positive until it attains some stable average positive potential, which is lower than that of the plate "I but is high enough to cause a drop in the output of the tube. If in accordance with my invention the inner wall of the bulb is treated so that it has a coefllcient of secondary electron emission lower than that of the usual glass bulb wall, the average number of secondary electrons emitted is less than the average number ofprimary electrons received, hence the wall cannot become positive. 0n the other hand the wall will repel the primary electrons if it becomes negative, hence the treated wall remains close to zero potential. With the bulb wall at zero potential the virtual resistance la in series with the shunt circuit is practically infinite. and there is no serious loss in the high gain circult.

What is claimed as new is:

1. An electron discharge tube comprising a highly evacuated glass envelope enclosing an electrode assembly which includes a cathode and cooperating tubular anode concentric with said cathode, and a coating of finely divided carbon in the form of an annular band on the inner walls of said envelope adjacent said anode and electrically insulated from said electrode assembly, said coating being in the path of stray electron streams from said cathode to said walls.

2. An electron discharge tube comprising a hermetically sealed glass envelope enclosing an electrode assembly which comprises a thermionic cathode and coaxial anode, said envelope having on a portion of its inner wall in registry with said electrode assembly a rough surfaced carbon coating which has less secondary electron emissivity than glass and which is electrically insulated from said electrode assembly by smooth surfaced insulating portions of said envelope.

3. An electron discharge tube comprising a hermetically sealed glass envelope enclosing an electrode assembly comprising a thermionic cathode and a coaxial plate surrounding said cathode, the inner wall of said envelope having at each end an annular zone of substantial width with an uncoated smooth surface and an intermediate annular zone, of a width substantially the same as the length of said plate with a rough surfaced coating of finely divided carbon insulated from said electrode assembly.

4. An electron discharge tube comprising a highly evacuated glass envelope enclosing electrodes including an indirectly heated cathode, an anode surrounding and coaxial with said cathode, and a plurality of grid electrodes interposed between said cathode and said anode, said envelope having on its inner walls a rough surfaced carbon coating in registry with and coextensive with said anode and insulated from said electrodes.

5. The method of reducing the secondary electron emissivity of the walls of a glass bulb below that of glass which consists in forming a coat of carbon on the inner walls of said bulb by applying a sooty flame in close proximity to said inner walls and cleaning said walls along an annular zone near each end of said bulb to leave an annular band of sooty carbon on the middle of the bulb.

6. The method of reducing the secondary electron emissivity from the bulb walls of a radio tube which consists in forming a coating of finely divided carbon on the inner walls of said bulb by spraying the inside of said bulb with an alcoholic suspension of lamp black, and removing the undesired portions of said coating by rotating suitably shaped brushes within said bulb.

'7. An electron discharge tube comprising a highly evacuated envelope enclosing an electrode assembly which includes a thermionic cathode and an imper'forate anode surrounding and concentric with said cathode, and a porous coating of finely divided carbon on the inner walls of said envelope having a matte surface and positioned to intercept stray electrons passing the ends of said anode from said cathode to said walls.

8. An electron discharge tube comprising a highly evacuated glass envelope enclosing an electrode assembly comprising a thermionic electron emitting cathode, a grid electrode, and an anode shorter than said cathode surrounding and coaxial with said cathode and said grid, said envelope having on a portion of its inner wall a rough surfaced annular coating of finely divided carbon with a matte-like surface overlapping the ends of said anode and spaced from said electrode assembly by uncoated portions of said walls.

9. An electron discharge tube comprising a hermetically sealed glass envelope having a reentrant stemcarrying an electrode assembly enclosed in said envelope and comprising a thermionic cathode and coaxial anode, said envelope having on a portion only of its inner wall in registry with said electrode assembly and out of registry with said stem a rough surface of finely divided carbon-which has lesssecondary electron emissivity than glass and which is electrically separated from said electrodes by uncoated portions of said glass walls.

HARRY C. THOMPSON. 

